Plants in nature establish symbiotic associations with microorganisms referred to as mutualists conferring benefits on the growth, survival and multiplication thereof. These microorganisms can be isolated, and their beneficial properties can be used in crops so as to improve yield thereof.
The genus Colletotrichum (Ascomyceto, teleomorpho Glomerella) comprises more than 60 species and species complexes. It is morphologically characterized by typically acervular conidiomata, with or without setae, having unicellular hyaline conidia which can be straight or curved, with a size preferably bigger than 12μ, usually granular. Conidia can also be formed from mycelium or other conidia (microcyclic conidia). Conidia forms appressoria when they germinate. Some species form stromas or sclerotia.
The genus Colletotrichum comprises species which are important crop pathogens and which are, because of that, the most common and best studied species. However, within the genus there are many species which have been reported as being endophytes or epiphytes which cause no damage to the host plant (referred to as commensal), or which can even be beneficial for the plant (mutualists). Hyde et al. (Fungal Diversity 39 (2009) 147-182) provides a thorough description of all the species from the genus Colletotrichum which are currently known, including a list of hosts cited in the literature and specifying the type of interactions established with each host (pathogenic, commensal or mutualist). The evidence cited in this publication indicates that, among the species or species complexes considered as pathogenic, there may be asymptomatic strains. There are even cases of strains which behave as pathogens, commensals or mutualists depending on the host in which they are inoculated. An example of this case is C. orbiculare, which behaves as a pathogen in curcubitaceae but may behave as a mutualist in tomatoes, conferring resistance to pathogens and to drought and promoting the vegetative growth of the plant when it is root inoculated.